【1. Introduction】

To achieve desirable plasma conditions for various applications such as material processing, the gas temperature might be one of the most important parameters. Generally, plasma with a gas temperature of a few thousand to millions of Kelvin is called thermal plasma, and it is used for light emission and material processing such as plasma spraying and welding. If the gas temperature is a few hundred to thousands of Kelvin, the plasma is called low-temperature plasma, and it is used for fluorescent lamps, ozonizers, and the surface fabrication of semiconductors such as deposition.

However, up to now, there have been few studies on plasmas with a gas temperature between room temperature (RT) and cryogenic temperature, or on their application. Generally, as shown in the following equation, the gas temperature decrease is equal to particle (atoms and molecules) kinetic energy decrease.

Here, m is the mass of the gas particle, v is the average velocity of the gas particle, kB is Boltzmann constant and Tg is the gas temperature. Relating to it, below RT, phenomenon such as phase transitions in many gas occurs due to the decrease in particle kinetic energy, and it remains to be studied how this decrease in particle kinetic energy below RT would or may affect plasmas and applications using those plasmas. Therefore, in this work, 'cryoplasma' with a gas temperature below RT (Tg ≦ 300 K) including plasma gas temperature below freezing point was developed and studied. The final goal in this work is generate cryoplasma, and to study the effect of decrease in particle kinetic energy to plasma by continuously decreasing its gas temperature below the room temperature, and contribute in developing the basics of new research field in plasma processing field through its study.

【2. Preparatory experiments】

Cryoplasma in open air was generated as a preparatory experiment (Fig. 1). To cool the plasma gas temperature, the gas temperature of operating gas was decreased by liquid nitrogen before introducing into electrodes part. DBD electrodes were employed for the generation method to prohibit excessive plasma gas temperature increase. The helium cryoplasma generated in open air accompanied the frost formation on the plasma generating part although, the generation was stable. It was confirmed by thermocouples and CFD-ACE+ simulation that the plasma gas temperature is below the freezing point of the water. Moreover, the plasma gas temperature was estimated from rotational temperature of N2 second positive system emission measured by optical emission spectroscopy (OES) (Fig.2), and it was confirmed that the gas temperature almost matches with the gas temperature measurement by thermocouples. Therefore, it was confirmed that N2 rotational temperature measurement using OES spectra is also considered as an effective way of estimating plasma gas temperature of cryoplasma besides thermocouples. The results of these preparatory experiments had shown the possibility of generation of cryoplasma in further low temperatures and measurements of the effect of change in particle kinetic energy on the plasma by continuously changing its gas temperature.

【3. Generation and diagnosis of helium cryoplasma (296 - 5 K)】

Cryoplasma was generated under continuous gas temperature control between RT and 5 K. To avoid the frost formation at electrodes part, the apparatus which can separate generating chamber from the outer chamber by vacuumed layer zone was employed. Two types of DBD electrodes were used to observe the generation appearance of cryoplasma. One was ac-planer DBD electrodes (Fig. 3) and the other was jet-type DBD electrodes (Fig. 4). In ac-planer DBD electrodes case, the discharge pattern changed upon decreasing gas temperature of inner chamber. Comparing with reaction-diffusion (RD) model which is used for explaining patterns forming in nature, it is assumed that gas temperature change is inducing the change in the spreading manner of activator and inhibitor, which results in pattern transitions. Due to the gas density increase upon decreasing the gas temperature, breakdown voltage of cryoplasma increased. From the stability of the generation, jet-type DBD electrodes were used for further measurements.

As the gas temperature decreases, emission spectra from N2 second positive system starts to diverge from theoretical spectra and rotational temperature cannot be estimated. So the gas temperature of the plasma is not also obtained. Since invalidity of estimation of plasma gas temperature using optical emission spectroscopy was verified at lower temperatures, plasma gas temperature of cryoplasma was calculated using heat transfer equation, assuming a simple model. Thus, it turned out that the maximum difference of the gas temperature at the plasma generating part and the temperature measured at thermal sensor is 1 K (Fig. 5). So, the temperature measured in the inner chamber by thermal sensor was regarded as the gas temperature of helium cryoplasma. Moreover, from the calculation, it was confirmed that under the certain condition, the smaller plasma, the better controllability of the cryoplasma gas temperature can be gained (Fig. 5).

Then the effect of change in particle kinetic energy on the plasma by continuously changing its gas temperature between RT and 5 K was measured electronically and optically. Below around 50 K, the tendency of the electron density and temperature change is similar to the gas density change (Fig. 6(a) and (b)). So macroscopically, these changes are related to the gas density increase. Besides that, the tendency of the change in electron density and temperature below around 50 K is also similar to the second virial coefficient decrease upon decreasing gas temperature, which is indicating the increase of Van der Waals force between the particles.Furthermore, since the electron coupling parameter of helium cryoplasma below Tg = 60 K increases, the effect of Coulomb force is increasing between electrons. Thus, considering the results from microscopic view, it can be assumed that helium cryoplasma is not just the plasma with an increased gas density but also the plasma under the effect of interparticle forces at lower temperatures. Under those circumstances, discharge mode changed from atmospheric glow (Fig. 7(b)) to atmospheric Townsend discharge (Fig. 7(c)) mode upon decreasing plasma gas temperature from 50 K. It is assumed that the generation of electrons due to collisions of helium metastables increased in the bulk gas upon decreasing plasma gas temperature. Finally, considering the result from both macroscopic and microscopic view at plasma gas temperature below 50 K, it can be assumed that plasma operation under three-body reaction due to high gas density and interparticle force due to less thermal heat favored the formation of He2* (Fig. 8). Therefore it can be also assumed that not only He2* but also larger helium clusters may exist in cryoplasma at plasma gas temperature below 50 K.

【4. Conclusion】

In this study, continuing to thermal plasma and low-temperature plasma, 'cryoplasma' with a third range of gas temperatures (Tg ≦ 300 K) including plasma gas temperature below freezing point was focused.The appearance, electron density and temperature, discharge mode and excited species of cryoplasma at various gas temperatures below RT all showed Tg dependent results. Therefore, through this study, it can be said that the plasma transitions were seen through the continuous particle kinetic energy decrease, which was expected at the beginning of this study in the motivation from pure science point of view. Especially at the gas temperatures below around 50 K, cryoplasma not only increases gas density but also interparticle forces start to affect the particles due to the decrease in particle kinetic energy. Bounding around that temperature, the results changed drastically compared with higher gas temperatures. Specifically, Te decreased whereas Ne increased, discharge mode changed into atmospheric Townsend mode and He2*appeared. On the other hand, from an application point of view, this study showed that reactions in bulk plasma changes as the gas temperature decrease, which results in the generation of different kind and amount of activated species. Therefore, cryoplasma may be utilized for controlling the reaction to gain expected species for material processing. Moreover, cryoplasma could be the source for cluster plasma application or even be an effective tool for material processing if optimum Te and Ne are gained by changing the generation methods. Pattern change and discharge mode change upon changing the plasma gas temperature may also be utilized in material processing in low temperature environment below RT.

This study is the starting point of study on cryoplasma. Results and discussions obtained through this study opens up the possibility of not only further basic studies of cryoplasma, but also many other underlying cryoplasma application studies in this gas temperature range, such as generation of cryoplasma for applications and actual application of cryoplasma, which will further contribute in the developing new research field in plasma processing field.

Fig. 1. Generation of helium cryoplasma in open air.

Fig. 2. Emission spectra of N2 second positive system at T = 200 K and theoretical fitting at 220 K.

Fig. 3. Generation of AC-planer DBD helium cryoplasma in chamber (RT to 78 K).

Fig. 4. Generation of jet-type DBD helium cryoplasma (RT to 5 K).

Fig. 5. Estimation of the plasma gas temperature increase depending on plasma sphere size with the same power consumption density (48 mW/cm(-3)).

Fig. 6. Gas-temperature-dependent (a)electron density and (b) electron temperature of helium cryoplasma jet co-plotted with helium gas density.

Fig. 7. Current and voltage (I-V) measurement results of helium cryoplasma jet; (a) RT, (b) 200 K and (c) 40 K.

Fig. 8. OES spectra of helium cryoplasma at 5 K.

本論文は、プラズマのガス温度が数千~数万Kの熱プラズマ、室温~数千Kの低温プラズマに続く室温以下のクライオプラズマに焦点を当てており、プラズマのガス温度を室温以下で連続的に低下させて発生したクライオプラズマの診断を行っている。そしてプラズマ中の粒子の運動エネルギー低下によるクライオプラズマの特性変化などを明らかにしている。

本論文は4章から構成されており、第1章では序論として、ガス温度が室温以下で制御できるクライオプラズマの研究に至った背景についてふれている。またそれら背景のもと、研究の目的を明示している。

第2章では、準備実験として、先ず、大気中でクライオプラズマの発生を行ったことについて述べている。プラズマのガス温度を冷却する手段として電極部分に導入する動作ガスであるヘリウムをあらかじめ液体窒素にて冷却を行っており、クライオプラズマの発生手法としてはプラズマの過剰な熱化を防ぐのに適している誘電体バリア放電(DBD)が用いられている。そしてプラズマガス温度低下に伴うクライオプラズマの様子を観察したところ、プラズマ発生部周辺に霜の付着が伴っていたが発生自体は安定であると確認している。プラズマのガス温度は2通りの方法で得ており、1つ目が直接熱電対をプラズマに挿入し、計測された温度とシミュレーションを用いてプラズマのガス温度を評価する方法で、この方法により大気中ヘリウムクライオプラズマのガス温度が氷点下になっていることが示されている。また2つ目は発光分光法により測定した窒素の回転スペクトルから窒素の回転温度を算出しそれをプラズマのガス温度と見なす方法で、熱電対による測定結果とほぼ良い一致を示している。よってこの大気中クライオプラズマの準備実験により、室温から極低温まで連続的にプラズマガス温度を制御できるクライオプラズマの発生及びその診断の可能性を示唆している。

第3章では、第2章に基づき、室温から5Kまで連続的にプラズマガス温度を制御できるクライオプラズマの発生とその診断について述べている。電極周りの霜の付着を防ぐために真空断熱層によって内部チャンバーを大気から隔離できる実験装置を用いており、平行平板型電極を用いると、プラズマのガス温度低下(室温~78K)に伴って電極内に生成されるプラズマのパターンが変化することを確認している。これはフィッツヒュー・南雲方程式を用いて数理的に発生させたパターンとの定性的な対応から、プラズマのガス温度低下によって主に放電を消す方向に形成される電場の拡散が抑制されることと印加電圧が上昇することに起因していると推測された。一方、ジェット型電極を用いたクライオプラズマの発生および診断では、プラズマのガス温度低下(室温~5K)に伴い雰囲気密度が増加し放電開始電圧が増加することが示されている。また、ガス温度が低温になるほど窒素発光スペクトル測定によるクライオプラズマのガス温度見積もりが有効でなくなるということを示している。そこで代わりに熱伝導の式を使って計算を行い、プラズマ発生部と原料ガスの測定をシリコンダイオードセンサーで行っている部分との温度差は最も温度上昇しやすい条件のもとであっても1K程度と見積もられている。すなわち、センサーで測定されている原料ガス温度をクライオプラズマのガス温度と見なして良いことを確認している。次に、プラズマガス温度低下に伴うプラズマの特性変化を調べるために電気学的、および、分光学的診断を行っている。電子密度と電子温度の結果よりプラズマガス温度が50K以下になるとミクロ的な視点からはクライオプラズマがファンデルワールスカやクーロンカなどの分子間相互作用力の影響下にあることが示唆されている。また、放電モードも50Kあたりを境に大気圧グローモードから大気圧タウンゼントモードへ遷移しているのが確認されている。クライオプラズマの発光分光の結果と合わせるとこの大気圧タウンゼントモードへの遷移はプラズマのガス温度低下に伴ってヘリウムの準安定励起種(He*、He2*)の衝突による電子生成が増加したことに起因すると考察している。また、プラズマガス温度低下に伴うHe2*の発光強度増加は雰囲気密度の増加というマクロ的な要因と、プラズマ中の粒子の運動エネルギー低下に伴う分子間相互作用力の増加というミクロ的な要因の両方に関連していると考察している。

第4章では、本研究の総括を述べている。

以上、本論文は、プラズマのガス温度が数千~数万Kの熱プラズマ、室温~数千Kの低温プラズマに続いて室温以下のクライオプラズマに焦点を当てており、プラズマのガス温度を室温以下で連続的に低下させて発生したクライオプラズマの診断を通してクライオプラズマ研究の新規性を見出している。さらにまた、クライオプラズマを近い将来プロセスへ応用することを見据えてその必要性および可能性を示している。ゆえに本論文はプラズマ材料科学において新しい研究分野の基礎づくりに貢献するものと判断できる。

なお、本研究の第2章は、石原大輔、Sven Stauss、崔允起、笘居高明、寺嶋和夫との共同研究であり、第3章は崔宰赫、Sven Stauss、笘居高明、寺嶋和夫との共同研究であるが、論文提出者が主体となって、分析および、検証を行ったもので論文提出者の寄与が十分であると判断する。

従って本論文に対して博士(科学)の学位の授与を認める。